In small internal combustion engines utilizing a carburetor, such as those engines in a lawnmower or a snowblower, cold temperature starting of the engine requires a more fuel-rich fuel-air mixture in the intake manifold of the engine to sustain the combustion reaction. In some engines, this is done by closing a choke valve, thereby partially choking off the air supply to the engine. As the engine warms up, the choke is no longer necessary because the increased temperatures in the engine help to sustain the combustion reaction and thus the choke is opened, allowing more air into the intake manifold. In many small engines, the choke valve is actuated manually.
Typically during warm engine restarts, the choke must remain open to start the engine and to prevent the engine from stumbling or stalling. During cold starts, if the choke valve is opened too soon, the engine stalls because the fuel-air mixture is not rich enough to sustain the reaction. If the choke remains on too long, the engine may also stumble and excessive hydrocarbon emissions and fouling of the spark plugs can occur.
Prior attempts at designing a small engine with automatic choking capabilities have been relatively unsuccessful, mostly because the designs of those chokes have been complex and thus not practical to manufacture at low cost in large numbers because the tolerances within the engine had to be very closely calculated and adjusted. Every part in the engine adds to tolerance variations that must be accounted for in the engines. The fewer parts in the engine, the easier it is to calibrate for the tolerance variations.